Ukraine may be a minor aviation market for the world's airlines, but the country's airspace is disproportionately valuable as a strategic crossroads linking some of the world's most important destinations.

Flights such as Malaysia AirlinesFlight 17—which U.S. officials say was shot down Thursday by a surface-to-air missile, killing all 298 onboard—was only one of many that crisscross Ukrainian skies. The airspace lies along direct routes between big markets like Scandinavia and Northern Europe, on one side, and expanding Middle...